1. Field of the Invention
The present invention relates to a measurement method and apparatus used to measure a numerical aperture (“NA”) of a projection optical system in an exposure apparatus.
2. Description of the Related Art
A conventional projection exposure apparatus projects a circuit pattern of a reticle (mask) onto a wafer or another substrate via a projection optical system in manufacturing fine semiconductor devices, such as a semiconductor memory and a logic circuit, using the photolithography technology.
The minimum critical dimension or a resolution transferable by the projection exposure apparatus is proportionate to a wavelength of the light used for exposure, and inversely proportionate to the NA of the projection optical system. The shorter the wavelength is and the higher the NA is, the smaller the resolution is. Along with the recent demands for fine processing to a semiconductor device, use of a short wavelength of the exposure light and a high NA scheme of the projection optical system are promoted. In particular, as the high NA scheme of the projection optical system proceeds, the NA matching in the exposure apparatus is important, and a highly precise NA measurement and adjustment of the projection optical system (an aperture shape and position of an aperture stop in the projection optical system) are increasingly demanded.
Conventionally, there are proposed some measurement methods of the NA of the projection optical system. Japanese Patent Laid-Open No. (“JP”) 3-65623 measures a luminance or light intensity distribution at the aperture stop position based on the light that has passed the aperture stop in the projection optical system, and calculates the NA of the projection optical system from the luminous or light intensity distribution measurement result. More specifically, as in the measurement method of JP 2005-322856, noises are removed from the light intensity distribution which is measured through integrations and smoothing, and the light intensity distribution is smoothed. The NA of the projection optical system is calculated from a local minimum value in a differential light intensity distribution curve that is derived from a differential process to the smoothed light intensity distribution.
Both JP 03-65623 and the measurement method of JP 2005-322856 are silent about the influence of the diffracted light generated from the aperture stop's edge in the projection optical system, on the light intensity distribution. When there are plural local minimum values in the differential light intensity distribution curve or plural inflection points in the measured light intensity distribution, JP 2005-322856 does not specify which inflection point corresponds to the NA of the projection optical system or whether the inflection point precisely corresponds to the NA of the projection optical system.
When the light is irradiated onto the aperture stop, the light that has passed the aperture stop and the light that has been diffracted at the aperture stop's edge interfere with each other. It is thus assumed that the light intensity distribution just after the aperture stop is the light intensity distribution that contains a diffraction fringe having a periodic relief pattern near the edge. The differential light intensity distribution curve derived from such a light intensity distribution has plural local minimum and maximum values. It is assumed that the measurement method of JP 2005-322856 detects the local minimum value in the differential light intensity distribution curve after smoothing the diffraction fringe as noises. Since the diffraction fringe provides information from the aperture stop's edge, the measurement method of JP 2005-322856 that smoothes the diffraction fringe as noises cannot precisely measure the NA of the projection optical system (aperture stop's aperture shape and position).